1. Field of the Invention
The present invention relates to a dispersion compensating fiber doped with a rare-earth element, and to an optical amplifying apparatus using the dispersion compensating fiber. In particular the invention relates to a dispersion compensating fiber with a limited rare-earth element doped region, and to an optical amplifying apparatus using this dispersion compensating fiber.
2. Description of the Related Art
In optical communication systems, for example, an optical transmission path comprising a single mode optical fiber (SMF) with zero dispersion in the vicinity of a wavelength of 1.3 .mu.m is used. Hence in the case where light with a central wavelength in the 1.55 .mu.m band is transmitted over long distances, there is a dispersion of around 18 ps/nm/km in the transmission path with respect to this 1.55 .mu.m band light, with the problem that when the signal is transmitted at high transmission speeds, the waveform becomes distorted.
To solve this problem, then for example there is a method wherein dispersion exactly opposite to the dispersion produced in the transmission path is given to the transmitted signal to thereby effect compensation. For the means for this dispersion compensation, for example, a dispersion compensating fiber (DCF) or a fiber grating or the like is generally used. In particular, the dispersion compensating fiber provides an excellent dispersion compensation device from the point of reliability, and stability with respect to external influences, and that dispersion slope can also be compensated. Such a dispersion compensating fiber can be obtained by changing the refractive index profile or the relative index difference, or the core diameter or the like of a single mode optical fiber.
FIG. 6 shows an example of dispersion characteristics of a slope compensation type dispersion compensating fiber.
In FIG. 6, for example, "a" indicates the dispersion characteristic of a single mode optical fiber having a length of 1 km (zero dispersion wavelength 1.3 .mu.m), while "b" indicates the dispersion characteristic of a dispersion compensating fiber having a length of 0.2 km. The single mode optical fiber has a positive dispersion slope with respect to an increase in wavelength, while the dispersion compensating fiber has a negative dispersion slope. The dispersion characteristic for the sum of the respective dispersion amounts is indicated by "c", showing a comparatively flat dispersion slope with the dispersion becoming zero in the vicinity of a wavelength of 1.55 .mu.m.
If a dispersion compensating fiber having such a dispersion characteristic is inserted into an optical transmission path, then the dispersion characteristic for the whole of the optical transmission path can be controlled to give a characteristic where the dispersion amount is approximately zero in the 1.55 .mu.m band, thereby obtaining a transmission signal with wave form distortion removed.
As an optical amplifying apparatus using a conventional dispersion compensating fiber, for example, the present applicant has proposed a device wherein a dispersion compensating fiber is inserted between two stage optical fiber amplifiers (refer to Japanese Patent Application No. 9-113882, or "Wideband Er-Doped Fiber Amplifiers for WDM Systems and Their Applications to Ultra Large Capacity Optical Transmission Experiments", Laser Research, vol. 2, No. 25, February 1997).
However in using a dispersion compensating fiber, there are many problems. One problem is that a dispersion compensating fiber has comparatively large insertion losses and these losses change according to the dispersion amount to be compensated. Another problem is that with a dispersion compensating fiber, since the mode field diameter is small, non-linear effects are more likely to arise than with normal transmission path optical fibers, so that if light is input at high power, the signal waveform deteriorates.
Regarding the former problem, a technique has been proposed to compensate for the losses due to dispersion compensating fibers, for example, by doping with a rare-earth element such as Erbium (Er) along the whole length of the core region of the dispersion compensating fiber, and supplying excitation light to this dispersion compensating fiber, to give an optical amplifying effect. Conventional techniques of this type are known for example from Japanese Unexamined Patent Publication Nos. 3-211530 and 6-11620.
With an optical communication system for transmitting signal lights over long distances, in order to compensate for the dispersion characteristics of the light transmission path, a relatively long distance dispersion compensation fiber is required. However if the insertion losses of such a long dispersion compensation fiber are compensated by the abovementioned conventional Er (or the like) doped dispersion compensating fiber, the following problems arise.
In the Er (or the like) doping region (the full length of the core region of the dispersion compensating fiber), in order to effectively obtain the optical amplifying effect, an excitation light of sufficiently high power must be supplied to the dispersion compensating fiber. For example, FIG. 7 shows aspects of the change in the signal light power inside a dispersion compensating fiber for the case where excitation light of differing power is supplied from the rear (output) side of a conventional dispersion compensating fiber which is doped with Er along the full length of the core region.
In FIG. 7, the abscissa axis shows the distance from the input end of the dispersion compensating fiber, while the ordinate axis shows the light power level. Moreover, level P1 is the threshold value for the input light power at which non-linear effects are produced in the dispersion compensating fiber. The signal light power for when excitation light is not supplied changes as shown by curve "a". As can be seen by comparing this with the change for the case of a normal dispersion compensating fiber which is not doped with Er (or the like) (dotted line), the inherent insertion losses for the Er (or the like) doped dispersion compensating fiber are greater than the insertion losses for the fiber which is not doped with Er (or the like).
The signal light power for when low power excitation light is supplied changes as shown by curve "b". In order to obtain the optical amplifying effect due to the induced emission phenomena from doping with Er (or the like), the power of the excitation light must be more than a certain level. However, since the excitation light incident from the rear is attenuated while being transmitted to the front inside the dispersion compensating fiber, then even with provision of the optical amplifying effect on the output side of the dispersion compensating fiber, along the way the signal light loses the amplification and is significantly attenuated. Therefore, if the power of the excitation light is low, the insertion losses of the dispersion compensating fiber cannot be sufficiently compensated.
In the case where high power excitation light is supplied so as to obtain the optical amplifying effect along the whole length of the dispersion compensating fiber, the signal light power changes as shown by curve "c". If such excitation light is sent to the dispersion compensating fiber, then the insertion losses of the dispersion compensating fiber can be compensated.
However, it is not easy to obtain an excitation light source of high output power. Moreover, doping with Erbium (or the like) along the whole length of the dispersion compensating fiber also has the disadvantage of high manufacturing costs.
Furthermore, even if an excitation light source of high output power is obtainable, if the power is excessive, then as shown by curve "d" in FIG. 7, because the signal light is amplified while the power level is comparatively high, the signal light power can exceed the level P1. In this case, there is the problem that non-linear effects occur so that the signal waveform deteriorates.